Electrochemical machining (ECM) and Electro-discharge machining (EDM) are two techniques used in industry for the machining of metals. In EDM, a DC voltage is applied to a drill electrode and the work piece is eroded by a spark formation in a gap between the drill electrode and the work piece. A dielectric liquid is usually forced into the gap between the electrode and the work piece.
In ECM, a drill electrode is placed in proximity to the work piece and an electric potential is placed across the drill electrode and the work piece. Electrolyte is forced into the gap between the electrode and the work piece, and work material is removed by electro-chemical action.
Commercially available EDM drilling machines, as opposed to EDM machining machines, may use water as the working fluid. In some cases, a non-conductive de-ionized water may be used, however, in some cases tap water may be used wherein the conductivity depends on the mineral content of the tap water. The EDM drilling process is not exactly the same as the EDM machining process. The EDM machining process uses a non-conductive dielectric, whereas in EDM drilling, a semi-conductive fluid may be used EDM machining has some similarity with ECM (Electro-Chemical Machining), which uses highly conductive electrolyte. The metal removal process is partly spark erosion and partly electro-chemical. Therefore, commercial EDM drilling machine uses a process in between that can be called an Electro-chemical Discharge Machining (ECDM).
Typically, for both ECDM and EDM, the drill electrode is hollow and the machining liquid (either the dielectric liquid or the electrolyte, depending upon the application) flows internally along the electrode, issuing through a hole, slot, or some other like aperture at the working face of the electrode. In ECDM, bubbles resulting from electrolytic dissolution cause a non-conducting region between the electrode and material, subsequently leading to an electrical discharge owing to a high electrical voltage applied to this non-conducting region.
Unfortunately, currently available ECDM and EDM tools are large, cumbersome, and have an inability to be used in confined spaces. Currently available ECDM tools and EDM tools are configured for use on work pieces that must be installed in a drilling machine such that the EDM or ECDM drill electrode is moved down towards the work piece, much in the same way as a drill is moved down in a drill press. Additionally, ECDM and EDM currently only drill holes around 6 mm in diameter, when a larger diameter drill hole may be needed to efficiently drill out certain hardware such as pins and screws.
As stated above, currently available EDM and ECDM tools are impossible or very difficult to use in confined spaces. An example of a confined space is the space around the rotor blades attached to a rotor of a turbomachine. Turbomachines include, but are not limited to: steam turbines, compressors, and gas turbines. Rotor blades often need to be removed from the rotor of a turbomachine. Such blade removal may be required, for example, to allow inspection, refurbishment or cleaning of the blades during scheduled maintenance or after a required shutdown of the turbomachine. A rotor for a turbomachine, such as a steam or gas turbine, typically has several rows of blades arranged along its periphery. Each row of blades comprises a circumferential array of blades spaced equally about the circumference of the rotor. Typically, each blade has a root portion by which it is retained in the rotor. Various blade root shapes have been utilized, such as firtree, dove-tail, etc. At assembly, the blade roots are axially slid into correspondingly shaped grooves formed in the rotor circumference. A locking device, such as a pin, is typically used to prevent the blade root from sliding out of the groove. During operation of the turbomachine, the pins may seize in their respective holes. Once these pins have seized, they are very difficult and time consuming to remove by using such known means as hammering or mechanical drilling. Part of the difficulty in removing these pins and tabs is that space is very limited between the hubs of a turbomachine rotor, thus making it very awkward if not impossible to drill out the pins and tabs. Additionally, the blades extend around a 360 degree interior of the turbo machine casing, making it difficult to position cumbersome tools to drill out all the pins.